60 



of snow evaporation is profoundly influenced by the wind. In our 

 western mountains, where the snows are exposed to dry winds, the 

 loss through evaporation is a large percentage of the total snowfall. 

 In the San Bernardino Mountains, snowfalls a foot in depth are 

 sometimes evaporated in two or three days without even moisten- 

 ing the soil. In so far as forests check the winter winds and pro- 

 vide shade, they lessen winter evaporation. This lessening of the 

 evaporation from snow surfaces, through the action of forests, is 

 seen in the fact that snows linger much later in spring in well- 

 wooded regions than in open areas. 



It appears, then, that forests materially retard evaporation, both 

 of soil moisture and of snow fall. 



DO FORESTS INFLUENCE TRANSPIRATION 1 



When land is covered with vegetation a certain amount of 

 the rainfall is taken up by the growing plants. A small part, 

 through chemical change, becomes incorporated into the plant, 

 but the larger part is returned to the atmosphere through transpi- 

 ration. Although those who have investigated this subject are by 

 no means in accord, there is reason to believe that considerable 

 difference exists in the amount of water taken up by the different 

 types of vegetation in the process of growth. On the whole, the 

 forest probably takes up less water from the soil than the average 

 agricultural crop. Risler, from a lengthy series of investigations, 

 reached the conclusion that forests actually take up less than one- 

 half as much water from the soil as the average agricultural crop. 



The above would lead one to infer that where the soil, if not 

 covered with forest growth, is clothed with grass or some other 

 low form of vegetation, the return of moisture to the atmosphere, 

 through evaporation and transpiration, or, in other words, the " fly- 

 ofl"," is less from the forest than from the open. But in regions 

 having a short wet season followed by a long dry one the return 

 of moisture to the atmosphere is probably greater from a forested 

 area, because in the open for a large part of the year there is very 

 little to evaporate, and the scanty .growth of grass and other low 

 forms of vegetation gives little opportunity for loss through trans- 

 piration. 



THE INFLUENCE OF FORESTS IN REGULATING THE RUN-OFF. 



Stream flow consists of both surface run-off and seepage run- 

 off". Although these two cannot be separately determined, total 

 run-off admits of accurate measurement. Surface run off may be 

 considered as flood water, while seepage run-off is that portion of 

 the drainage which gives the streams a sustained flow. It is evi- 

 dent that any factor which decreases the surface or superficial run- 

 off and increases the seepage run-off is of the utmost importance 

 in regulating the flow of streams. 



